Electroretinography and suicidal behaviors: a systematic review

Background Electroretinogram (ERG) is one of the tools used to investigate the electrophysiological underpinnings of mental health illnesses and major clinical phenomena (e.g., suicide) to improve their diagnosis and care. While multiple studies have reported specific ERG changes among individuals with suicidal behaviors, we know of no review that has been done to characterize their findings to inform future research. Methods This review included available literature concerning ERG and suicidal behaviors. The paper’s first section briefly overviews the theoretical basis of ERG and neurotransmitters involved in suicidal behaviors. The second section describes the findings of a review of studies reporting ERG findings among individuals with suicidal behaviors. Results Most reviewed studies reported normal amplitude and implicit time of the a-waves, but the latency in individuals with suicidal behaviors was lower than normal. Additionally, the b-waves amplitude was reduced, but the implicit time and latency were increased. The b-a amplitude ratio and oscillatory potential were decreased. Conclusion Despite identifying certain ERG correlates with suicidal behaviors in the existing studies, there is a need for adequately powered and methodologically robust studies to advance clinical translation. Supplementary Information The online version contains supplementary material available at 10.1186/s12888-023-05453-w.


Introduction
Suicide is a public health issue of concern, with over 700,000 individuals dying by suicide yearly, but proportionally more people demonstrating suicidal behaviors [1,2].The etiological underpinnings and ramifications of suicide are often construed with the biopsychosocial There are many electrophysiological techniques that have the potential to detect suicidal behaviours.In particular, several studies have explored techniques related to testing brain electrical activities using electroencephalography (EEG) including quantitative EEG, polysomnographic examination and quantitative sleep EEG, auditory evoked potential loudness, and other eventrelated potentials) to detect suicide [10].The findings obtained with these techniques are sometimes coupled with radiological testing [9], which support the idea that executive function directly impacts emotion regulation in individuals with suicidal behavior.When emotion regulation is disrupted, individuals with suicidal behavior turn to maladaptive procedures that reduce their ability to cope with emotional stress.Emerging evidence has also shown important correlates of suicidal behavior using electroretinography (ERG) -involves the measurement of the electrical response of the eye's light-sensitive cells, called rods and cones.Notably, the ERG has shown a reduction of the b-wave latency among individuals with suicidal behaviors [7].
Despite the potential benefits of using these noninvasive electrophysiological methods to detect suicidal behaviors, introducing these methods into routine clinical practice has been problematic, given their economic costs and dependence on the operators' skills.For example, EEG is limited to measuring electrical potentials from the area of cortical gray matter and hippocampi, while electrical potentials from subcortical structures, such as the basal ganglia and brainstem nuclei, cannot be measured by EEG because their electrical potentials do not reach the surface of the head [11].Comparatively, majority of the available electrophysiological research and reviews have explored the role of EEG and radiological testing in suicidology, but few have focused on ERG correlates of suicidal behaviors.Hence, this review focuses on the ERG correlates of suicidal behaviors.The review is divided in two sections, first we provided a brief overview of the theoretical basis of ERG and neurotransmitters involved in suicidal behaviors, and the second section described results from a review of available studies about suicidal behaviors and ERG.

Overview of the theoretical basis of ERG and neurotransmitters involved in suicidal behaviors Electroretinography
The ERG is a specialized electrophysiologic tool that records the electrical responses of retinal cell types, including photoreceptors (rod and cones), bipolar and amacrine cells, and ganglion cells [12,13] (Fig. 1).ERG is particularly useful for determining the generalized retinal function amongst individuals.ERG detects the retinal responses through the placement of a few small electrodes around the eye area and a succession of brief flashes of light.Common parameters measured by ERG include the amplitude and latency of a and b-waves, while less common parameters include a photopic negative response (PhNR) and oscillatory potentials (OP) (Table 1).These parameters will be described below in further detail (Fig. 2).
The International Society for Clinical Electrophysiology of Vision (ISCEV) is a global association that standardises clinical protocols for electrophysiological examination.The first ISCEV guidelines came out in1989 [14,15], and have been regularly updated.The latest update from 2022 [16] outlines six standardized full-field ERG protocols for isolating the electrical activity of the rods and cones.The protocols are named based on the eye's adaptation state (whether dark-adapted or light-adapted) and the stimulus strength (light intensity).Four protocols are darkadapted, and two are light-adapted [12].An extended photopic on-off protocol for ERG has been used to assess retina function in studies analyzing psychiatric conditions [9].The specifications of the protocol are as follows: (1) the stimulus duration is between 150-200ms, Fig. 1 Retina cells.Representation of the retinal network and its different pathways for visual information processing.Visual information is processed by the 'columnar unit, ' which includes photoreceptors, bipolar cells, and retinal ganglion cells.Horizontal and amacrine cells modulate the synaptic signaling between photoreceptors and bipolar cells and between bipolar cells and retinal ganglion cells.Retinal ganglion cell axons initially form bundles of unmyelinated fibres at the retinal nerve fibre layer (RNFL), which will then exit the eye and form the myelinated optic nerve.Rod and cone photoreceptors mediate two distinct pathways of light responses: scotopic and photopic vision.In particular, the cone pathway can be additionally distinguished according to ON and OFF responses or waves, which are interconnected and differentially modulated by glutamate neurotransmission (2) the stimulus should be white or chromatic, with the same-coloured background, (3) the strength of the stimulus must be between 150 and 350 cd m − 2 with the background luminance at 30 cd m − 2 , and (4) the rate of stimulus presentation should be less than or equal to two per second, with an inter-stimulus interval of greater than or equal to five seconds [17].The protocol emphasises the on-and-off response to light and produces various waves based on the retina cells' response.The waves include a, b, and d waves.Both a and b waves are on-responses while d-wave constitute the off-response.The a-wave (the initial negative deflection) corresponds to the early hyperpolarization of the rod and cone photoreceptors.The b-wave is the positive deflection following the a-wave.B-waves originate from the depolarization of inner retinal glia and bipolar cells.However, off-bipolar and horizontal cells may influence their amplitude and shape [17].The off-response or d-wave is a positive polarity component in response to stimulus offset [17].The d-wave initial rapid phase originates from off-bipolar cell activity, but cone photoreceptors contribute to the later slow phase, and on-bipolar cells act in an opposite polarity direction.See Table 1.
Other aspects of the waves include Oscillatory potentials (OPs) and PhNR.OPs are visible at greater signal intensities and represent high-frequency rhythmic wavelets on the rising slope of the b-wave.OPs reflect the electrical activity of inner retinal feedback synaptic circuits, namely amacrine cells and some vascular functions.PhNR is the negative deflection that follows the b-wave and represents light-adaption.The PhNR originates from the retinal ganglion cells in response to a brief flash.
The waves are analyzed based on amplitude, implicit time, latency, and b-wave to a-wave ratio [20].The amplitude is the maximal light-induced voltage (electrical response) generated by the different retinal cells.The a-wave amplitude measurement starts from the baseline of the ERG to the negative trough of the a-wave, and the b-wave amplitude starts from the trough of the a-wave to the subsequent b-wave peak.Like the a-wave, the PhNR amplitude measurement starts at the baseline of the ERG to the negative trough of the PhNR.Implicit time, or time-to-peak, is the time needed for the electrical response to reach maximum amplitude.Implicit time is measured from the stimulus onset to the peak of the corresponding wave component.Thus, the implicit time reflects the rate of electrical signal conduction.Unlike implicit time, latency is the time from stimulus onset to response onset.The b-wave to a-wave ratio provides an index of inner to outer retinal function.However, the accuracy of the findings of the ERG is affected by various factors [20].

Factors affect the findings of an ERG
In their work, Asanad and Karanjiah [20] have previously outlined factors that interfere with ERG findings.They are outlined as follows: (i) Use of non-standardized testing conditions, such as sub-optimal lighting, recording environment, pupil size, flash intensity, and altering duration of light or dark adaptation; (ii) Uncorrected refractive errors; (iii) Ocular media opacification; (iv) Electrode-based artifacts due to poor contact with skin

Neurotransmitters within the retina responsible for the ERG findings
Based on human and animal studies, various neurotransmitters have been identified in the retina, mainly in the amacrine cells [18].The following are neurotransmitters in the Amacrine cells: GABA, Glycine, Acetylcholine, Dopamine, Serotonin, Substance P, VIP, Somatostatins, and Nitric oxide.However, the dominant neurotransmitter is Glutamate [18,21].The oscillatory potentials reflect the activities of the amacrine cells [20].The bipolar cells have receptor channels for glutamate, but their axonal endings have receptors and channels for GABA (A, B, and C types), dopamine (D1), and glycine.The ganglion cells have diverse receptors as bipolar cells with additional acetylcholine receptors.Polymorphism in genes is responsible for the neurotransmitters, their receptors, and channels identified in the retina, and are hypothesized as the underpinning mechanisms modulating the connection between neurotransmitters' physiological changes and mental health conditions.That said, few studies have explored the molecular relationship between neurotransmitters' physiological changes in the retina and brain-related conditions or mental health illness.
In view of the potential link between neurotransmitters in the brain and the eyes, ERG has successfully detected retina-related changes for various mental health conditions, including depressive disorders, panic disorders, ADHD, autism spectrum disorder, eating disorders, and schizophrenia [22].Also, ERG has been used to detect changes related to suicidal behaviors among individuals with the major depressive disorder [7].

Neurotransmitters correlated with suicide or suicidal behaviors
Almost all neurotransmitters in the retina correlate with suicide [23,24].For instance, a recent review on neurotransmitter systems related to suicide using postmortem studies reported that monoamines, glutamate, GABA, and endocannabinoids, neurotransmitters, enzymes, or receptors are correlated with suicide [24], albeit monoamines have been relatively well studied [25].In fact, one of the first described molecular changes in individuals who are suicidal was the correlation between low serotonin levels and 5-hydroxy indoleacetic acid (5-HIAA) in cerebrospinal fluid (CSF) with suicidal behaviors [23,[26][27][28].Further research has also supported this link: including the finding that altered levels of serotonin and serotonin signalling are correlated with suicidal behaviours [29], and binding of the serotonin 1A BP F in the raphe nuclei was correlated with the lethality of a suicide attempt.Specifically, a higher lethality suicide attempts was linked with higher levels of binding of the serotonin 1A BP F in the raphe nuclei compared to lower lethality attempts [30].

Neurotrophic factors correlated with suicide or suicidal behaviours
In addition to the neurotransmitters, there are neurotrophic factors that may influence neuronal connections in the retina, which have been correlated with suicide.One example is the differential expression patterns of the BDNF receptor TRKB between individuals who die by suicide and controls [29].With neurotransmitters and neurotrophic factors in the retina correlating with suicide or suicidal behaviors, ERG is a potential non-invasive biomarker for suicidal behavior, although more studies are needed to establish this concept.

A review of existing studies on ERG
A few published reports highlighted findings about the use of ERG to study suicidal behaviors, but no review has been conducted to synthesize these findings [7,[31][32][33][34][35][36][37][38][39][40][41][42][43][44][45].Here, the available literature has proven very useful for new research by showing the use of ERG waveform changes in detecting suicidal behavior.It is with this background that we design this study to describe eligible studies on this theme and comment on the opportunities they provide as well as their limitations.Overall, we focused this review to summarize all the consistent results that have been described while acknowledging that studies with contradictory results reflect the complexity of this area of research.

Methods
The present review included existing reports on suicide, suicidal behaviors and ERG.The present review followed the PRISMA guidelines [46].All article types were included, and there was no restriction to the language in which the studies were published.The following databases were used: Ovid databases (i.e., Embase, APA PsycInfo, Ovid Emcare, Ovid Medline, and Epub Ahead of Print), PubMed, Web of Science, and CINAHL.The following keywords were used with assistance from the institutional librarian (Kaitryn Campbell) to develop a search strategy: (i) electroretinography (Electroretinography/ OR (electroretinography* OR electro-retinograph*).tw,kw,kw,id.)and (ii) suicidal behaviors (exp Suicide/ OR suicid*.tw,kw,kw,id.).
Following the removal of duplicates, all identified articles were screened independently by pairs of researchers via Covidence.Screening was based on the following process (i) title and abstract, (ii) full text, and (iii) data extraction and cleaning (See Fig. 3).The following information was extracted by a pair of authors: Author, title, year of publication, study design, study group, countries where the study was conducted, sample size, medication before ERG, suicidal behaviors assessed, a method for assessing suicidal behaviors, ERG testing done, ERG findings, conclusion about ERG findings.

Study selection
A total of 50 studies were identified from the different databases; 17 were duplicates.The remaining 33 articles' titles and abstracts were screened, and 19 papers were considered eligible.After a full-text review, 14 articles were included in the current study.Two full texts could not be retrieved [31,32].JBI critical appraisal tools were used to assess the quality of the included study (Supplementary File 1).Overall, studies did a good job of describing patient conditions upon arrival and of describing the ERG findings.Many studies [29,[32][33][34][35][36][37] did not include an intervention or treatment, as assessing treatment was not the intent of these studies.However, this exclusion does not impact the results of the following findings as this review was only interested in ERG measures following suicide.

Characteristics of the study participants
The age of the participants in the case reports and case series ranged between 19 and 59 years.However, the majority of the studied cases included individuals who were below 50 years, a finding that resonated with the cross-sectional study (mean age 41.0 ± 11.4).Most of the studies had female individuals who attempted suicide, and four had male individuals [33,37,43,45].Three studies reported a history of mental illness among the participants, and they included bipolar affective disorder [34], major depressive disorder [7,34], and borderline personality disorder [38].A total of 19 participants included in this review had attempted suicide.

Periods ERG was performed
A total of six studies followed up patients from the first day of the suicide attempt [37-39, 41, 43, 44], while five studies performed ERG five months after the suicidal attempt [33,35,36,42,45].

ERG wave findings
The findings ranged from having completely extinguished waves to fluctuating waves following a suicidal attempt.The following waves were affected as shown in Table 3.
Based on the included papers, most studies reported normal amplitude and implicit time of the a-waves.Still, the latency was lower compared to individuals without suicidal behaviors.The b-waves amplitude was reduced, but the implicit time and latency were increased.The b-a amplitude ratio and oscillatory potential were decreased.

Discussion
The finding from the various included studies indicates a correlation between ERG waveform changes and suicidal behaviors.However, most of the included studies were case reports and a case series.More robust studies with advance methods are needed to better understand the electrophysiological changes associated with suicidal behaviors and their clinical translation.Future studies should address the following methodological issues, taking into consideration the observations highlighted.
• The findings in the included studies might have been cofounded by hypoxia-related neuronal defects in the retina, which could influence changes in neurotransmission and the waves displayed by the ERG.It is necessary to address this issue by controlling for this effect or excluding individuals with possible hypoxia-related suicide attempts due to hanging, use of methanol, rat poison (such as Vacor), quinine, sildenafil, among others.Also, because the OP waves depend on some vascular response in the retina, the wave's shape is affected by vascular diseases such as diabetic retinopathy on ERG [41].Therefore, we recommend screening individuals for diabetes and potential vascular diseases that may affect the retina.• Individuals who attempt using sildenafil may also be excluded due to PDE5 inhibition in bipolar and ganglion cells and PDE6 inhibition at the photoreceptors that affect the ERG wave forms [43].• Individuals who have attempted suicide with quinine-related compounds should also be excluded because the vast available literature among animals and humans' studies showed its effect on the ERG findings [38,44].However, this should be done in consideration that the studies in humans were case reports, and no controlled study was assessed.In -Days 3 and 6: a-wave amplitude was normal while b-waves were enlarged (more for the left than the right eye).
-6 months: b-wave amplitude decreased substantially by this date.
-The a-wave implicit time reached normal by day 2, while the b-wave implicit time increased up to day 12.
-OPs, compared to controls, was completely extinguished on day 9, improved on days 10 and 12 (though still decreased compared to the control) and remained diminished by six months.
- -At both the scotopic and photopic SF -The a-wave amplitude exceeded the b-wave amplitude as the b-wave had selective reductions in both conditions (electronegative waveforms).
-For the scotopic SF -normal a-wave amplitudes, but reduced b-wave amplitudes.
-For the scotopic SF -normal a-wave implicit times and faster than normal b-wave implicit times -Phototopic SF-abnormal b-wave with a delayed implicit time.
-Scoptopic OPs -abnormal with a single positive peak (corresponding in time to the positive peak in original waveform) - -The delayed a-wave (prolonged implicit time) when there was no light perception.Indicates evidence of an acute effect on photoreceptors cell layers.
-The later decline in b-wave shows inner layer damage.This may be through vascular constriction or ischemia due to quinine.-Quinine must affect the initial electrically silent part of the retina that don't contribute to the fERG waveforms e.g., the ganglion cells.This is due to having a normal early ERG in the presence of severe visual dysfunction.
-The inner retina was mainly affected because the abnormal pattern ERG and reduced OPs during the same period in the patient.
-"The pattern ERG was markedly reduced, and no pattern visual evoked response (VER) could be recorded." -Case 2: The P50/ N95 ratio was significantly elevated and the fERGs were normal in both eyes.
-Normal fERG findings.and 142-days post-ingestion.-The amplitude and pattern for the scotopic and photopic responses in both eyes were diminished.
-A significant reduction in both a-wave and b-wave indicating a primary insult to the photoreceptors.
-During treatment, the ERG responses showed mild improvement in the a-wave amplitude in both eyes in the maximal scotopic conditions.Following treatment discontinuation, the a-wave of both eyes decreased.
-The b-waves had no similar pattern and continued to deteriorate during follow-up.
-Only the left eye single flash photopic responses demonstrated some improvement during treatment but decreased after treatment ceased.
-ERG showed transient increase in a-wave amplitude.A-wave decreased after treatment.o This improvement may be attributed to a positive effect of 9-cis beta-carotene treatment on the photoreceptors and the retinoid cycle -A limited, slow recovery of the ERG responses after they were almost completely absent over a 2-year followup.Thus, a short-term increase in the a-wave amplitudes may be the direct effect of treatment.
-These changes may represent the natural course of ERG responses in quinine toxicity.
-The lack of consistent increase in ERG b-wave responses during treatment o This may be because 9-cis beta-carotene impacts photoreceptors, which are not responsible for b-waves six months after suicide attempt -Recording of the right eye done at DA 0.01 cd s m-2 showed a reduced amplitude a-wave; reduced b/a wave amplitude ratio at DA 3.0 and DA 10.0 cd s m-2.
-The OPs on the DA 3.0 wave showed reduced amplitudes.
-Light-adapted ERG also showed a reduced b/a wave amplitude ratio along with a reduced amplitude 30-Hz flicker.
-The dysfunction was post-phototransduction (at the level of the inner bipolar cells of the retina) due to having an electronegative ERG in the presence of a normal electrooculography (EOG).
-Reduced OP waves suggest amacrine cells dysfunction -The cause of the inner retina cell dysfunction could be due to hypoperfusion.
Chen   addition, not all the different parameters of the ERG were explored and affected.For instance, the b/a wave ratio and OP among others were not explored, despite evidence suggesting that they can detect changes in neurotransmitters in the retina, which could potentially be correlated with suicidal behaviors.• The use of 9-cis-beta-Carotene in managing quinine-related ERG changes [38] may indicate that the vitamin can be used in suicidal behavior management.Other studies have suggested that 9-cis-beta-Carotene is significantly helpful in managing suicidal behaviors and depression [48][49][50].
The effect is related to the antioxidant properties of the vitamin [49].The study did not check the impacts of the vitamin on OP and other important ERG parameters to make a satisfactory conclusion on its importance on the neurotransmitters or waves in the retina [38].• Individuals who use carbon monoxide poisoning should also be considered excluded because of the potential effects it had on the ERG findings [33].However, the findings were reversed and normal after three weeks following the suicidal attempt.

Study limitations
The findings of this review should be interpreted with caution for the following reasons.(i) majority of the studies are case reports and lack sufficient statistical power to confirm the actual effect or changes in the ERG waves caused by suicidal behaviors.It also makes it hard to distinguish the effects of cofounders to clarify whether the effect on ERG wave is due to medications, suicidal behaviors, or psychiatric illness.(ii)There was no actual link between neurotransmitters in the retina and specific regions responsible for suicidal behaviors in the brain.This makes the conclusions on the findings about ERG correlates of suicidal behaviors difficult since no study has explored such results to implicate specific brain regions We recommend that future studies explore this link.(iii) The OP waves are not specific to retina neurotransmitters and may not be specific to inhibitory or excitatory cells in the amacrine cells.The amacrine cells have many neurotransmitters that are correlated with suicidal behaviors.The lack of specificity of the waves to distinguish between the neurotransmitters makes detecting suicidal behaviors difficult.We recommend that future researchers determine the effect of individual neurotransmitters on the OP waves and other waves of the ERG.This will advance the science of eliciting potential biomarker that is more specific and reliable.(iv) Only few studies have explored ERG use among individuals with suicidal behaviors.These studies are limited because of cross-sectional design and use of small sample, and causality can not be inferred.We recommend future prospective studies s with larger sample sizes to enable the detection of the small changes in the different wave measurements, such as latency, and amplitude, among others, (v) Distinguishing between the severity of suicide from depression is complex using ERG because of the high level of similarities between the neurotransmitters involved in the two conditions.Future researchers show study suicidal behaviors among individuals with and without depression to detect the ERG differences in the two conditions using a control.(vi) Many studies were performed months after the suicidal attempt, and it is hard to determine the effect or correlates of suicidal behavior on the ERG.A followup study proximal to the suicidal attempt may yield better results for at least six months post attempt since ERG changes related to suicide were observed at six months.
(vii) Most studies involved females, and significant differences between genders may need to be explored.Also, the extreme of ages were not studied.Therefore, future studies should consider matching the populations studied and controlling for variables that may confound, such as gender, age, and medication use, among others.(viii) The ISCEV standards for ERG use are updated regularly, making ERG comparisons difficult as the case studies used for this review span decades, including before the first ISCEV standards were published.[41,45] Oscillatory potential Decreased [39,41,44,45] and were completely extinguished after [39,41] Overall ERG patterns Reduced [43,44] Normal [33] Note: Some studies refer to latency and implicit time as the same thing, defining them both as the time it takes to reach the trough of the a-wave and peak of the b-wave [47].While one study measured latency [7], it is unclear whether they were measuring the time from stimulus to onset of response or the time from stimulus to the trough and peak of the waves.

Conclusion
The majority of the studies reported normal amplitude and implicit time of the a-waves, but the latency was lower compared to individuals without suicidal behaviors.The b-waves amplitude was reduced, but the implicit time and latency were increased.The b-a amplitude ratio and oscillatory potential were decreased.This may indicate the effects of suicidal behaviors on the inner nucleus layers of the retina with a potential reduction of the depolarization of the inner retina Muller, bipolar cells, and amacrine cells.An effect may be related to some of the neurotransmitters within the layers.Among these neurotransmitters, many are involved in suicidal behaviors.While some promising results have been highlighted, more research work is needed to translate the evidence from existing literature into clinical practice and to advance the potential utility of ERG findings as biomarker in suicidology.Notably, further studies are needed to explore the relationship between ERG waves, retina neurotransmitters, and suicidal behaviors.

Table 1 ERG wave forms Wave Functional retina cells represented Neuronal activity Possible neurotransmitters
The classical ERG graphs showing the different waves.

Table 2
Findings from the different studies

Table 3
Wave forms for individuals with suicidal behaviors